1. Field of the Invention
The present invention relates, in general, to an etching method for a zinc selenide (ZnSe) polycrystalline substrate. In particular, it relates to an etching method for a ZnSe polycrystalline substrate whereby a smoothly etched surface can be obtained.
2. Description of the Background Art
With the miniaturization of electronic components and electronic devices used in mobile telephones, personal computers, etc., there has been increasing demand for finer and higher speed drill processing. The application of a diffractive optical element (DOE) has been promoted as a key device to satisfy this demand.
Unlike conventional optical components utilizing refraction and/or reflection, the DOE utilizes optical diffraction and directly controls the phases, thereby becoming an optical component, of which a wide range of applications including, beam splitting, for example, can be expected FIG. 9 shows an example wherein the DOE is utilized in drill processing using a carbon dioxide gas laser. Use of the DOE makes it possible to split one processing laser beam into multiple spots and simultaneously drill a plurality of holes, thus realizing high-speed processing of minute holes. Application examples of the DOE to other laser processing are shown in FIG. 10. ZnSe is excellent in infrared transmittance and is used as material of optical elements for a carbon dioxide gas laser. In general, as for ZuSe having a diameter of 1 or 2 inches and a thickness of several millimeters, which is commonly used as an optical element, that of polycrystal is used as opposed to that of single crystal in consideration of cost. In most cases, high purity ZnSe polycrystals synthesized by a chemical vapor disposition (CVD) method are used.
FIG. 8 is a diagram showing the general steps for manufacturing a DOE using ZnSe polycrystals.
Step A: Synthesizing a ZnSe polycrystal from Zn and H2Se.
Step B: Cutting the ZnSe polycrystal, forming a ZnSe polycrystalline substrate 1, and polishing the surface thereof
Step C: Forming a resist layer 2 on the ZnSe polycrystalline substrate 1.
Step D: Selectively irradiating light 4 by means of a photomask 3 and forming a printed pattern on the resist layer 2.
Step D and Step E: Developing the resist layer 2 and forming a resist pattern 5.
Step F: Performing reactive ion etching (RIE) for the ZnSe polycrystalline substrate 1 by means of the resist pattern 5 and forming a pattern 1a on the substrate 1.
Step G: Removing the resist pattern 5.
Step H: Forming an anti-reflection (AR) coating layer 6 on the ZnSe polycrystalline substrate 1.
By the RIE method used in step F of FIG. 8, a chemical reaction occurs between radicals generated from the reactive gas and ZnSe on the substrate surface, generating by-products. These by-products are then removed by sputtering. Thus the etching progresses.
Hydrocarbon-based gas, which is commonly used for a signal crystal, is known as etching gas of the RIB method. However, if hydrocarbon-based gas is used as the etching gas, the etching rate depends heavily on the crystal direction of the polycrystal grains. As a result, as shown in FIG. 12, the surface of the ZnSe polycrystal becomes rough, deteriorating the optical characteristics of the DOE.
It is assumed that, owing to the mechanism shown in FIG. 11, the etching rate depends heavily on the crystal direction of the polycrystal grains in the case where hydrocarbon-based gas is used as the etching gas.
When the hydrocarbon-based gas is used, the gas reacts with the ZnSe at the substrate surface and generates by-products of metal-organic compounds such as dimethylzine, dimethylselenide and the like. They have a high vapor pressure and a strong tendency to desorb from the substrate surface simultaneously with the generation. And the ratio to be removed by sputtering is small. Namely, the etching rate greatly depends on the reaction between the radicals and ZnSe on the substrate surface.
With respect to polycrystals, atomic density on the forefront surface is different depending on the crystal direction of the crystal grains, while the radicals generated from the reactive etching gas uniformly reach the surface. As a result, etching progresses slowly for a crystal grain having a high atomic density on the surface compared to a crystal grain having a low atomic density. Since the etching rate is different depending on the crystal direction of the crystal grains, unevenness is created among individual crystal grains after etching.
The present invention provides an etching method for a ZnSe polycrystalline substrate which has been improved so that a smoothly etched surface can be obtained.
According to the present invention, a ZnSe polycrystalline substrate is etched by a reactive ion etching method by means of chlorine-based gas which does not include a hydrocarbon group BCl3 gas is preferably used as the chlorine-based gas. In addition, inert gas or gas that does not react to ZnSe may be mixed in the above chlorine-based gas. Ar gas can be used as the inert gas. Furthermore, the above-described reactive ion etching is preferably performed at 0.5 Pa through 1 Pa. Radio frequency (RF) power can be utilized for activating the gas.